Show combined memory usage in parallel mode

[janedbal]

Closes phpstan/phpstan#4683

Looks like this:

$ php bin/phpstan clear-result-cache -q && php bin/phpstan -v
Note: Using configuration file /usr/src/myapp/phpstan.neon.
 1486/1486 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100% 56 secs/56 secs

 [OK] No errors                                                                 

Took about 1.35 GB memory

[janedbal]

I dont think there is a better option. Still imo much better than before. It seems working well when comparing with system memory used.

[janedbal]

Tested against linux ps and it gives very good results, almost equal.

[janedbal]

This is the point of time where phpstan consumes most memory (I believe in >99% cases), so I add main thread memory at that specific time.

[ondrejmirtes]

Thank you!